Question
NEET 2022 (paraphrased): Regarding factors affecting photosynthesis, which of the following statements is correct? (a) Blackman’s law refers to above a threshold, more light doesn’t help (b) light saturation is characterised by rate is set by the factor in shortest supply (c) CO₂ compensation point — CO₂ level where photosynthesis = respiration (d) optimum temperature has no role in factors affecting photosynthesis
Solution — Step by Step
This option swaps the definitions. Blackman’s law is actually rate is set by the factor in shortest supply, not what (a) claims. So (a) is out.
Similarly, light saturation means above a threshold, more light doesn’t help. Option (b) is mismatched. Out.
This one reads correctly: CO₂ compensation point — CO₂ level where photosynthesis = respiration. Match with NCERT wording confirms it.
optimum temperature — 25–35°C for most C₃ plants — plays a real role in factors affecting photosynthesis, so option (d) is factually wrong.
Correct answer: (c) CO₂ compensation point — CO₂ level where photosynthesis = respiration.
Why This Works
NEET PYQs on factors affecting photosynthesis rely on definition swapping. The examiner takes two correct facts and pairs them with the wrong terms. If you know each fact cold, the MCQ dissolves in 20 seconds.
Alternative Method
For definition-swap MCQs, don’t read all four options first. Read option (a), decide true/false, then (b), and so on. This prevents confusion between similar-looking options.
NEET reuses the same fact set every 2–3 years with different distractors. Master the four NCERT facts on factors affecting photosynthesis listed here and you’ll handle almost any MCQ on this chapter.
Common Mistake
Students second-guess themselves when they see a familiar option listed as “wrong” in the stem. Trust the elimination — if you know the definition, stick with it.
Keep a one-page sheet of definitions for every NEET chapter. Revise only that sheet in the last 48 hours before the exam.